A shock absorber of the present invention has a function of absorbing shock to a passenger of the vehicle at the time of collision or the like by disposing in an element of the vehicle. PLT1 and the like disclose this type of shock absorbers that have a hollow, double-wall structure obtained by blow-molding a thermoplastic resin and where the ends (end walls) of recessed ribs formed from the front and rear walls are joined together to improve shock absorbency.